In the classical dual pressure process for air distillation, cooled air is supplied to a high pressure rectifier. The HP rectifier overhead reboils the bottom of a lower pressure nitrogen removal column, and also evaporates the oxygen product. The rectifier liquid products are routed to the LP column--kettle liquid as feed, and LN.sub.2 as overhead reflux (directly injected). The required process refrigeration may be obtained by either of two approximately equivalent methods: work expansion of part of the supply air to an appropriate height of the LP column, or work expansion of part of the HP rectifier overhead N.sub.2 to exhaust pressure (approximately the LP column overhead pressure).
The recovery of O.sub.2 is determined by the ability of the LN.sub.2 reflux at the LP column overhead to rectify O.sub.2 out of the N.sub.2 exhaust. For the classical process described above, there is more than adequate LN.sub.2 reflux available even after satisfying refrigeration requirements to achieve very high O.sub.2 recoveries, on the order of 98% or more. However, whenever any process enhancement is introduced in order to achieve increased byproduct recovery, increased purity, or reduced energy consumption, there is usually also a corresponding reduction in the LN.sub.2 available for reflux, and hence a decrease in O.sub.2 recovery.
For example, LN.sub.2 coproduct or pressurized GN.sub.2 coproduct obviously cause a mole-for-mole reduction in LN.sub.2 reflux availability. Increasing the O.sub.2 delivery pressure by LOXBOIL results in less air supplied to the HP rectifier, and hence less LN.sub.2 available as reflux. LOX coproduct decreases the HP rectifier reflux, which is LN.sub.2. Argon byproduct requires an auxiliary argon rectifying section of the LP column, which communicates at its bottom with the vapor and liquid streams of the LP column between the argon stripping section and nitrogen stripping section of the LP column. The argon rectifier is conventionally reboiled by exchanging latent heat with part of the kettle liquid. This raises the quality of the feed to the LP column, and hence increases the need for LN.sub.2 reflux. As a final example, low-energy triple-pressure processes such as disclosed in U.S. Pat. Nos. 3,688,513, 4,507,134, and 4,578,095 also supply one part of the air to the HP rectifier, and gasify part of the kettle liquid, thereby decreasing the LN.sub.2 reflux availability on two counts.
In summary many different hypothetical improvements to air separation end up being less advantageous than first supposed, or even disadvantageous, due to the reduction in LN.sub.2 reflux availability and the resulting decrease in O.sub.2 recovery. What is needed is a method or apparatus which makes more LN.sub.2 reflux available than in conventional practice, without additional power requirement or excessive increase in equipment cost. That is one primary objective of the disclosed invention.
It is known to expand part of the supply air to produce refrigeration, and drive a warm end compressor with the expansion work. U.S. Pat. No. 2,666,303 discloses such an apparatus in which overhead N.sub.2 from the LP nitrogen removal column is compressed, thereby lowering LP column pressure below atmospheric pressure. West German patent application No. 2854508 by Rohde, published 6/19/80, discloses such an apparatus wherein the warm compressor compresses the expansion air before cold expansion.
Other prior art references involving similar companders include U.S. Pat. Nos. 3,261,168, 4,133,662, and Russian Patent No. 756150.
It is known to warm HP rectifier overhead N.sub.2 to near ambient pressure, compress it with an externally powered compressor, re-cool it, and expand it back to HP rectifier overhead pressure, thereby providing the relatively large amount of refrigeration necessary in liquefaction cycles. One example is U.S. Pat. No. 3,319,427. It is also known to use a warm-end compressor driven by the expander to pick up part of the external compression duty, as disclosed in U.S. Pat. No. 4,099,945.
It is known to boil liquid nitrogen in a latent heat exchanger at the overhead of the argon rectifying section of a dual pressure column (U.S. Pat. No. 3,729,943) and of a triple pressure column (U.S. Pat. No. 4,578,095).
None of the above prior art references addresses or solves the problem of providing increased LN.sub.2 reflux and hence increased O.sub.2 recovery in a flowsheet wherein a discharge stream of pressurized N.sub.2 gas must be expanded for refrigeration.